In recent years extensive development has been in progress on optical recording media having a recording layer that is made of phase change material that enables repetitive recording. A number of phase change materials has been studied, including chalcogenide alloys such as Sn—Se—Te alloys, Sb—Te alloys, In—Se alloys, Ga—Te alloys, and Ge—Te alloys. In CD-RWs and 4×DVD+RWs, in particular, Ag—In—Sb—Te alloys are now in practical use that are obtained by addition of several different elements to Sb—Te alloys. Ag—In—Sb—Te alloys have several advantages that they readily undergo vitrification, produce uniform recording marks, are less likely to undergo composition change such as segregation and phase separation during multiple recording and erasing operations, and readily undergo initial crystallization.
As recording media have become widespread, there is an increasing expectation that further high-speed recording would be available. In 4×DVD+RWs, high-speed recording/erasing of up to 14 m/s has been made possible by the use of Ag—In—Sb—Te alloys in their recording layers. Phase change optical recording media utilize difference in light reflectivity between crystalline and amorphous phases in the recording layer. Because it is necessary for the phase change optical recording media to rapidly crystallize amorphous marks in order to achieve high-speed recording/erasing, there is a demand to develop phase change materials with high crystallization rates. For example, Ag—In—Sb—Te alloys with a higher Sb content for increased crystallization rates have been used as materials high-speed recording. Increased Sb contents, however, lead to reduction in the crystallization temperature in the recording layer and thus undesirably make the amorphous mark's shape non-uniform. For this reason, 4×DVD+RW (maximum recording speed=14 m/s) is the practical limit in this method of increasing the Sb content for increased crystallization rates.
Patent Literature 1 discloses an optical recording medium in which either the first protective layer or the second protective layer contains ZnO in an amount of 50 mol % or more. The technology disclosed in Patent Literature 1 aims to provide a high-density optical recording medium with excellent repetition durability by preventing cross-erasing and amplitude reduction upon repetitive recording. Patent Literature 1 discloses that the crystalline state of ZnOx, a main ingredient of the recording layer, is changed by addition with N, which aims to prevent mechanical deformation of the recording layer due to, for example, generation of stress in the layer by adjusting the hardness of the protective layer material. Patent Literature 1 discloses as recording layer materials chalcogenide alloy materials such as Pd—Ge—Sb—Te alloys, Nb—Ge—Sb—Te alloys, Pd—Nb—Ge—Sb—Te alloys, Ni—Ge—Sb—Te alloys, Ge—Sb—Te alloys, Co—Ge—Sb—Te alloys, In—Sb—Te alloys, Ag—In—Sb—Te alloys, and In—Se alloys. These materials are different from recording layer materials described in the present invention, which contain Sb as a main ingredient. Thus, the invention of Patent Literature 1 is distinct from the present invention in terms of the object and constitution.
Patent Literature 2 discloses an optical recording medium in which either the first protective layer or the second protective layer is formed of ZnO-BN, and aims to provide a phase change optical disc that features a small degree of amorphous mark deformation and exhibits excellent recording sensitivity and thermal stability. The recording layer of the optical disc uses a chalcogenide compound, and the recording layer material is different from that of the present invention that contains Sb as a main ingredient. Thus, the invention of Patent Literature 2 is distinct from the present invention in terms of the object and constitution.
Patent Literature 3 discloses an invention that aims to provide a high-density recording medium that, upon high-density recording, offers excellent signal quality (e.g., in terms of jitter) and excellent repetitive recording characteristics. Patent Literature 3 also discloses that, in an optical recording medium in which a first protective layer, a recording layer, a second protective layer and a reflective layer are arranged over a substrate, the electric resistance of the second layer is set to 3×10−3 Ωcm or less as a means of improving productivity and increasing recording sensitivity. Patent Literature 3 discloses In—Sn—O, Zn—Al—O, Sn—O, etc., as the materials of the second protective layer, but fails to describe the crystallinity of the second protective layer. Because a chalcogenide compound is mainly used in the recording layer, the recording layer material is different from that of the present invention that contains Sb as a main ingredient. Thus, the invention of Patent Literature 3 is distinct from the present invention in terms of the object and constitution.
Patent Literature 4 discloses an invention that aims to provide a high-density recording medium that, upon high-density recording, offers excellent signal quality (e.g., in terms of jitter) and excellent repetitive recording characteristics. Patent Literature 4 also discloses that, in an optical recording medium in which a first protective layer, a recording layer, a second protective layer and a reflective layer are arranged over a substrate, the electric resistance of the first layer is set to 3×10−3 Ωcm or less for the purpose of improving productivity and increasing recording sensitivity. Patent Literature 4 discloses In—Sn—O, Zn—Al—O, Sn—O, etc., as the materials of the first protective layer, but fails to describe the crystallinity of the these materials. Because a chalcogenide compound is mainly used in the recording layer, the recording layer material is different from that of the present invention that contains Sb as a main ingredient. Thus the invention of Patent Literature 4 is distinct from the present invention in terms of the object and constitution.
Patent Literatures 5 to 7 each discloses an optical recording medium using a ZrO2 oxide as a crystallization facilitating layer. These Patent Literatures describe the effect of facilitating crystallization, but fail to describe the crystallinity of the ZrO2 oxide. Because a chalcogenide compound is mainly used in the recording layer, the recording layer material is different from that of the present invention that contains Sb as a main ingredient. Thus, the inventions of Patent Literatures 5 to 7 are distinct from the present invention in terms of the object and constitution.
A phase change optical disc is generally obtained by depositing a thin film over a transparent plastic substrate provided with a particular groove. Plastic material used to manufacture this substrate is often polycarbonate, and injection molding is often used for the formation of the groove. The thin film deposited onto the substrate is a multilayered film, which is basically formed of a first protective layer, a recording layer, a second protective layer, and a reflective layer, which are stacked in the order in which light passes through the disc for recording or reproduction of information. Oxides, nitrides, sulfides, etc., are used for the first and second protective layers. Among these compounds, ZnS—SiO2 obtained by combining ZnS with SiO2 is often used. As the materials of the protective layer that is in contact with the recording layer, amorphous materials, such as ZnS—SiO2, have been generally used, because the protective layer comes in contact with the recording layer to significantly affect the recording layer and hence the use of a crystalline protective layer makes amorphous marks prone to crystallization, causing a considerable reduction in the archivability.
Even Sb—Te alloys, particularly those with a high Sb content, exhibit low crystalline temperatures and hence resultant amorphous marks are prone to undergo crystallization. Accordingly, the use of crystalline materials for the manufacture of a protective layer significantly affects the recording layer that is in contact with the protective layer, leading to a problem that amorphous marks disappear by crystallization even in an environmental test at 70° C.
It was established in the course of development of high-speed recording optical recording media that while the optical recording medium, for which high-speed recording of over 8× for DVD is intended, offered excellent jitter values—a measure of the variations in mark shapes over time—many errors occurred. This phenomenon is one that has never been seen in conventional discs designed for low-speed recording. This phenomenon is more likely to occur upon low-speed recording on a optical recording medium that is capable of high-speed recording, and thus remains an important issue to be addressed. The present inventors investigated the cause of the frequent occurrence of this error and found that this is due to the fact that marks are sometimes generated that are extremely shorter in length than normal marks.
[Patent Literature 1] Japanese Patent Application Laid-Open (JP-A) No. 11-353707
[Patent Literature 2] Japanese Patent Application Laid-Open (JP-A) No. 09-138974
[Patent Literature 3] Japanese Patent Application Laid-Open (JP-A) No. 11-185294 [Patent Literature 4] Japanese Patent Application Laid-Open (JP-A) No. 11-185295
[Patent Literature 5] Japanese Patent Application Laid-Open (JP-A) No. 2004-178779
[Patent Literature 6] Japanese Patent Application Laid-Open (JP-A) No. 2004-013926
[Patent Literature 7] Japanese Patent Application Laid-Open (JP-A) No. 2004-005767